System for hydrogen thermal-electrochemical conversion

ABSTRACT

A system for converting heat energy into electricity includes a conversion cell comprising a pair of spaced-apart electrodes having an electrolyte therebetween. The electrolyte is selected to pass negatively-charged hydrogen ions and to inhibit the passage of atomic hydrogen and positive hydrogen ions. Inducing a flow of hydrogen through the cell, a current may be generated between the electrodes as electrons are gained by the hydrogen as it enters the cell and lost by the hydrogen as it leaves the cell. In the preferred embodiment, hydrogen flow is induced by reacting the hydrogen leaving the cell with lithium or sodium to form the metal hydride. The metal hydride is then thermally decomposed to release the hydrogen and the molten metal to be recycled to the cell. In this way, the thermal energy used to decompose the metal hydride is converted into electrical energy by passing the hydrogen through the conversion cell.

This is a division of application Ser. No. 897,243 filed Aug. 18, 1986,now U.S. Pat. No. 4,692,390.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the conversion of heat energyinto electrical energy, and more particularly to a method and system fortransporting hydrogen ions through a selective electrolyte under achemical potential gradient to produce electricity.

2. Description of the Background Art

The conversion of chemical energy to electrical energy may beaccomplished in a variety of ways. Most commonly, electrochemical cellsand batteries rely on redox reactions involving the transfer ofelectrons from the substance being oxidized to the substance beingreduced. By carrying out the reaction in such a way that the reactantsdo not come into direct contact with each other, it is possible to causethe electrons to flow through an external circuit where they can be usedto perform work.

Although invaluable for a number of applications, electrochemical cellsdo suffer from certain drawbacks. In particular, such cells have afinite life resulting from the exhaustion of the reactants. Althoughmost cells can be recharged by applying a reverse-polarity voltageacross the electrodes, such recharging requires a separate electricalsource and prevents the continuous operation of the cell over indefiniteperiods.

To overcome these problems, fuel cells were developed. In general, fuelcells operate by passing an ionizable species across a selectiveelectrolyte which blocks passage of the non-ionized species. By placingporous electrodes on either side of the electrolyte, a current may beinduced in an external circuit connecting the electrodes.

The most common fuel cell is the hydrogen-oxygen fuel cell wherehydrogen is passed through one of the electrodes while oxygen is passedthrough the other electrode. The hydrogen and oxygen combine at theelectrolyte-electrode interface to produce water. By continuouslyremoving the water, a concentration gradient is maintained to induce theflow of hydrogen and oxygen into the cell. Fuel cells of this type havebeen particularly valuable in manned space flights where they not onlyprovide relatively large amounts of electricity, but also supplydrinking water for the personnel.

Despite their usefulness, fuel cells of the type described above sufferfrom a number of disadvantages. First of all, the fuel cells require acontinuous supply of reactant in order to continue to produceelectricity. Related to this, the cells also produce a continuousproduct stream which must be removed. Although disposal of the waterproduced by hydrogen-oxygen fuel cells is seldom a problem, the removalof the product of other fuel cell systems is not always as simple. Thesecond problem relates to the selection and maintenance of the porouselectrodes. Electrodes must be permeable to the reactant speciesentering the cell. Over time, however, such porous electrodes frequentlybecome fouled and plugged so that migration of the reactants through themembrane is slowed. Such slowing results in the reduced production ofelectricity. Third, the selection of an appropriate electrolyte is notalways easy. The electrolyte, which may be a solid electrolyte, mustrapidly transport the ionized species in order to increase the currentproduction. Frequently, the limited migration of the ionized speciesthrough the electrolyte is a limiting factor on the amount of currentproduced.

For these reasons, it would be desirable to provide fuel cells which donot require a continuous source of reactants in order to operate. Inparticular, it would be desirable if the fuel cells could operate withreactants which are regenerated by means of an alternate energy source,preferably heat. Such thermoelectric conversion cells will preferablyutilize electrodes and electrolytes which do not become fouled orplugged and which provide for rapid migration of the ionizable species.Finally, such thermoelectric conversion cells will display high currentto weight ratios allowing for their utilization in applications wherevolume and weight are critical, such as space flight.

Certain thermoelectric conversion cells have been proposed. See. e.g.,U.S. Pat. No. 3,458,356, where molten sodium is induced to flow across asolid electrolyte by a pressure gradient induced by a temperaturegradient. The electrolyte is chosen to selectively pass sodium ions, anda current is generated as sodium atoms lose electrons on entering theelectrolyte and gain electrons on leaving the electrolyte. The cell isworkable, but suffers from plugging of the porous electrodes required topass sodium ions. Moreover, diffusion of the sodium ions through thesolid electrolytes is relatively slow, limiting the amount of currentavailable from the cell.

Thermally regenerative fuel cells are also described in U.S. Pat. Nos.3,357,860 and 3,119,723. The following patents are also of interest:U.S. Pat. Nos. 3,014,048; 3,031,518; 3,192,070; 3,338,749; 3,368,921;3,511,715; 3,817,791; 4,049,877; and 4,443,522.

SUMMARY OF THE INVENTION

According to the present invention, electricity is produced by passinghydrogen gas through an electric conversion cell comprising a pair ofspaced-apart electrodes having an electrolyte therebetween. Theelectrolyte is chosen to selective pass ionized hydrogen and blocknon-ionized hydrogen, and electric current is generated as the hydrogenloses electrons at one electrode and gains electrons at the otherelectrode. Thus, a useful current is obtained by connecting an externalcircuit across the two electrodes.

The use of hydrogen as the sole ionizable species greatly simplifies thedesign of the cell as a variety of solid electrodes are permeable tohydrogen and suitable for use. Blockage and plugging of electrodes, asencountered in virtually all prior fuel cell applications, is of noconcern with the conversion cell of the present invention. Similarly,migration of hydrogen through the electrolyte is rapid, allowing veryhigh current densities based on the area of the electrodes. Thus, theconversion cell of the present invention provides for a very high energyoutput which does not substantially diminish over time.

Electrical generation using the conversion cell just described, ofcourse, relies on maintaining a continuous hydrogen concentrationgradient across the cell. In the preferred embodiment, the concentrationgradient is provided by continuously introducing hydrogen gas to a firstof the electrodes, while reacting the hydrogen gas with a molten metalat the second electrode to produce a metal hydride. The metal hydridemay then be removed from the second electrode and thermally decomposedin order to regenerate the hydrogen and molten metal. In this way, thecells of the present invention can be used in a system for continuouslyproducing electricity from heat. Because of the temperatures requiredfor a thermally decomposing metal hydrides, the system is particularlyuseful for converting high temperature heat sources, such as coolantfrom liquid metal cooled nuclear reactors. The system would also beparticularly suitable for electrical generation wherever mechanicalgenerators are impractical, such as in spacecraft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating the general operational principlesof the present invention.

FIG. 2 is a flow chart illustrating a particular embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a system 10 for converting heat energy from asource Q into electricity will be described. The system 10 includes aconversion cell 12 comprising a first electrode 14 and second electrode16 having an electrolyte 18 contained therebetween. The electrodes 14and 16 are held in a spaced-apart relationship within a vessel 20 whichwill be constructed to handle the rigorous operating temperaturesdescribed in more detail hereinbelow. The electrodes 14 and 16 will besealed to the inner wall of vessel 20 so that the electrodes and vesseltogether define a chamber for holding the electrolyte 18.

The vessel 20 and associated piping (as described below) may beconstructed of suitable metals, such as tungsten and molybdenum, orfiber reinforced ceramics. If metal construction contraction isemployed, all conductors, including the electrodes 14 and 16 must beelectrically insulated from the vessel 20 so that the electrodes remainisolated from each other.

The electrodes are composed of a solid metal which is chemically inertwith hydrogen and which allows relatively fast hydrogen penetration.Suitable metals include nickel, palladium, vanadium, zirconium, andniobium, and the like. As illustrated in FIG. 1, the electrodes 14 and16 are flat plates which are spaced-apart in a parallel manner, but thegeometry and dimensions of the electrodes may vary widely. The areas ofthe electrode(s) will depend on the desired amount of current generationand may vary from several cm² to several m² or larger. The electrodesshould be as thin as possible consistent with structural integrity andthe ability to conduct the expected current densities. Such thinelectrodes provide minimum resistance to hydrogen diffusion.

The electrolyte 18 is provided to selectively pass hydrogen ions (andinhibit the passage of non-ionized hydrogen) between the electrodes 14and 16. Suitable electrolytes include alkali metal salts and alkalineearth metal salts mixed with a metal hydride in an amount from about 5to 20%, typically about 10%. Conveniently, the electrolyte will bemaintained in a liquid state by elevating the temperature. To lower thenecessary melting point, eutectic salt mixtures may be employed.Suitable eutectic mixtures include lithium chloride and potassiumchloride, lithium iodide and potassium iodide, calcium chloride andcalcium hydride, and the like. Suitable metal hydrides include lithiumhydride and sodium hydride. Such electrolytes have been found to rapidlypass negatively charged hydrogen ions, while suitably blocking thepassage of uncharged and positively charged hydrogen ions.

The spacing of electrodes 14 and 15 and consequent volume of electrolyte18 is not critical. It is necessary only that sufficient electrolyte 18be present in order to effectively inhibit the passage of non-ionizedhydrogen and positively charged hydrogen ions. Typically, a spacingbetween electrodes 14 and 16 in the range from about 1 to 5 mm isdesired, more typically being about 2 mm.

The system 10 further includes a thermal decomposition vessel 26 whichreceives heat from a source Q. The decomposition vessel 26 receives astream of molten metal and metal hydride 28 through conduit 30 from alower chamber 32 in the vessel 20. The metal hydride 28 is thermallydecomposed to hydrogen and molten metal. The hydrogen flows to vessel 20through an overhead conduit 34 while the molten metal returns to thelower chamber 32 of vessel 20 through a lower conduit 36. The hydrogenenters an upper plenum 38 in the vessel 20 where it is evenlydistributed across the upper surface of electrode 14. A concentrationgradient exists across the cell 12 as hydrogen passing through the cellis continually depleted in the lower plenum 32. Thus, a driving forceexists for inducing a flow of hydrogen across the cell 12, and hydrogenis able to diffuse through the first electrode 14 until it reaches theinner face with electrolyte 18. At that point, the non-ionized hydrogenis unable to penetrate the electrolyte. However, by externallyconnecting the electrodes by means of an external circuit 40, electrons(generated as discussed hereinbelow) are able to flow from the secondelectrode 16 to the first electrode 14. There, negative hydrogen ionsare produced by the following equation:

    H+e.sup.- =H.sup.-

The negatively ionized hydrogen atoms are thus able to pass through theelectrolyte 18 reaching the interface with the second electrode 16. Asthe hydrogen ions enter the electrode 16, the electrons are lost,regenerating the non-ionized hydrogen and providing a source for theelectrons which pass through the circuit 40 to the upper electrode 14.In this way, it will be appreciated that electrical current capable ofproducing useful work is generated. The non-ionized hydrogen then passesinto the plenum 32 where it is able to contact the molten metal 42 whichhas been recirculated from decomposition vessel 26, as describedpreviously. The hydrogen rapidly reacts with the molten metal to producemetal hydride which is then circulated back to decomposition vessel 26through conduit 30. It will be appreciated that the molten metal 42 atthe bottom of vessel 20 is maintained at a lower temperature than themetal hydride in the decomposition vessel 26, favoring the formation ofmetal hydride.

The system just described is closed except for the input of heat Q andthe output of electricity through external circuit 40, and can thus forma mechanical system converting heat energy to electricity.

Referring now to FIG. 2, a more detailed system for producingelectricity according to the method of the present invention will bedescribed. Where possible, FIG. 2 will employ reference numeralscorresponding to those employed in FIG. 1.

The source of heat in the system 50 of FIG. 2 is a coolant stream from aliquid metal cooled nuclear reactor 52. The coolant is pumped through anexchange conduit 54 by an electromagnetic pump 56. The decompositionvessel 26 will employ either LiH or an NaH as the metal hydride,depending on the temperature of the liquid metal coolant employed.Typically, LiH will be employed for higher coolant temperatures in therange from about 1000° to 1400° K., while NaH will be employed for lowercoolant temperatures in the range from about 600° to 1000° K. At theappropriate temperature T₁, the metal hydride will break down intohydrogen, which passes through the overhead conduit 34 to vessel 20, andmolten lithium or sodium, which passes through a conduit 60 to a heatexchanger 62. The heat exchanger 62 lowers the molten metal to atemperature T₂ where the metal may again combine with hydrogen to form ametal hydride. The cooled molten metal then passes through conduit 64 tothe lower plenum 32 of vessel 20 where it is able to react with hydrogenpassing from the second electrode 16 of cell 12.

Hydrogen from decomposition vessel 26 passes to the upper plenum 38 ofvessel 12 through conduit 38. There, the hydrogen enters a plurality oftubular electrodes 70 which extend downward from a support plate 72. Thetubular electrodes 70 are composed of the same materials describedpreviously as suitable for electrodes, and provide an increasedelectrode surface area to enhance the permeation of the hydrogen intothe electrolyte 18 between the support plate 72 and second electrode 16.As described before, the electrodes 70 are isolated from electrode 16,but connected to electrode 16 through external circuit 40. As thehydrogen passes through the tubular electrodes 70, the hydrogen atomscombine with electrons released from electrode 16. The negativelycharged ions are able to migrate through the electrolyte 18 rapidly andarrive at electrode 16 where the electrons are given up. The non-ionizedhydrogen entering plenum 32 through electrode 16 combines with themolten metal which is at the lower temperature T₂. The metal hydride isthen pumped through conduit 30 by means of an electromagnetic pump 82 todecomposition vessel 26 where it is again heated to temperature T₁ anddecomposed into hydrogen and molten metal.

The operational temperature ranges for the preferred LiH and NaH systemsof the present invention are as follows:

    ______________________________________                                        Metal                                                                         Hy-        T.sub.1 (°K.)                                                                          T.sub.2 (°K.)                               Metal  dride   Broad     Narrow  Broad  Narrow                                ______________________________________                                        Lithium                                                                              LiH     1000-1400 1100-1300                                                                             500-800                                                                              500-600                               Sodium NaH      600-1000 700-900 400-600                                                                              400-450                               ______________________________________                                    

In operation, systems of the type just described can achieve highcurrent densities based on the electrode areas. The flow rate of liquidmetal hydride required will depend on the flow rate of hydrogen, andwill be sufficiently rapid to maintain a very low partial pressure ofhydrogen in the lower plenum 32, typically in the range from about 10⁻⁵to 10⁻⁴ mmHg. The voltage generated by the cells will depend on thetemperatures employed, while the current will depend on the temperature,electrode area as well as the circulation rate of the liquid metalhydride.

Although for foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

What is claimed is:
 1. A thermoelectric conversion cell comprising:apair of spaced-apart solid electrodes having an electrolyte therebetweenchosen to selectively pass ionized hydrogen atoms; means for supplyinghydrogen gas to a first of said electrodes; means for supplying areactant which reacts with hydrogen gas to a second of said electrodesso that a chemical potential created across the electrodes, whereby theflow of hydrogen across the electrodes induces a current flow betweenthe electrodes.
 2. A cell as in claim 1, wherein the electrodes arecomposed of a solid metal selected from the group consisting of nickel,palladium, vanadium, zirconium, and niobium.
 3. A cell as in claim 1,wherein the electrolyte is selected from the group consisting of amixture of lithium hydride and lithium chloride, and a mixture ofcalcium hydride and calcium chloride.
 4. A cell as in claim 1, whereinthe reactant is a molten metal which reacts with hydrogen to form ametal hydride.
 5. A cell as in claim 4, wherein the means for supplyinghydrogen gas comprises a means for thermally decomposing the metalhydride formed by reaction of hydrogen and the molten metal, which meansfor thermally decomposing also supplies the molten metal.
 6. Athermoelectric conversion system comprising:a cell including a pair ofspaced-apart solid electrodes having an electrolyte therebetween chosento selectively pass ionized hydrogen atoms; an external circuitconnecting the electrodes; means for thermally decomposing a metalhydride into hydrogen gas and molten metal; means for directing thehydrogen gas to a first of said electrodes; means for cooling the moltenmetal; and means for directing the cooled molten metal to a second ofsaid electrodes so that the metal reacts with hydrogen which has passedthrough the cell to reform the metal hydride, whereby hydrogen passingthrough the cell causes an electric current to pass through an externalcircuit connecting the electrodes.
 7. A system as in claim 6, whereinthe electrodes are composed of a metal selected from the groupconsisting of nickel, palladium, vanadium, zirconium, and niobium.
 8. Asystem as in claim 6, wherein the electrolyte selectively passesnegatively charged hydrogen ions.
 9. A system as in claim 8, wherein theelectrolyte is selected from the group consisting of a mixture oflithium hydride and lithium chloride, and a mixture of calcium hydrideand calcium chloride.
 10. A system as in claim 6, wherein the means forthermally decomposing includes a vessel heated by a nuclear reactor. 11.A system as in claim 6, further comprising means for recycling thereformed metal hydride to the means for thermally decomposing.